Computation of Time-Resolved EPR Spectra of Systems Exhibiting Electron Spin Polarization Complicated by Magnetization Transfer

Jäger, Martin; Norris, James R.

doi:10.1021/jp0135149

Cited by 10 publications

(7 citation statements)

References 26 publications

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“…The time developments of the computed quantities of ( M Z,1 + M Z, - 1 ) and ( M Z,1 − M Z, - 1 ) were convoluted with the response functions determined by the width (16 ns) of the π/2 microwave pulse . Several time-resolved EPR studies have suggested that homogeneous ET reactions between quinone anions and the dissolved neutral quinone molecules affect the decay time of the multiplet ( M Z,1 − M Z, - 1 ) contributions. ,, However, even if homogeneous ET were to occur, the effect needs not to be taken into account in this study, because both the net and the multiplet contributions were simulated with a common, single T 1 parameter in eq 3 as seen in Figure . These results indicate that the homogeneous ET rate is much slower than the original spin−lattice relaxation times of quinone anion radicals in the solvents used in this study.…”

Section: Resultsmentioning

confidence: 99%

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

et al. 2004

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Nanosecond pulsed electron paramagnetic resonance spectroscopy is applied to characterize exponential decay constants ( ) of the squared electronic coupling matrix element (V DA 2 ) in transient, solvent-separated radical ion pairs (RIP) composed of quinone anions and several cation radicals in aprotic liquid solutions of N,Ndimethylformamide, DMSO, and benzonitrile. The distance dependence of singlet-triplet energy splitting (2J) is shown to be described by in V DA for charge-recombination processes. We show that the radical pair mechanism (RPM) electron spin polarization (P RPM ) is quite sensitive to . The value is characterized by using the stochastic Liouville equation to fit the experimental P RPM values. The values (from 0.8 to 1.0 Å -1 ) manifest that V DA is governed by the superexchange mechanism mediated by the intervening solvent molecules from a result that the increases with increasing the tunneling energy gap (∆G eff ) for solvent oxidation or reduction in several intermolecular electron-transfer systems. We propose a simple threedimensional model of V DA , in which the through-solvent tunneling pathways are exponentially increased with the increase in the intermolecular distance in bulk, condensed media. This model explains the ∆G eff dependence of , including the data previously reported on the charge-transfer reactions both in liquid and frozen (77 K) solutions. Effective solvent-solvent coupling is estimated to be V B ≈ 850 cm -1 at a mean nearest-neighbor distance of 5.7 Å. This relatively large magnitude of V B may agree with dynamical amplifications of the effective coupling by low-frequency motions of the mediators as reported in charge-transfer reactions in biological systems. (Balabin, I. A.; Onuchic,

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Section: Resultsmentioning

confidence: 99%

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

et al. 2004

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“…The X-band TREPR measurements were carried out by using a magnet and console (Varian E-112) equipped with a microwave bridge (ER 041MR, Bruker, Billerica, MA) in which a microwave preamplifier was added to enhance the time resolution (Ϸ25 ns) and signal sensitivity (51). Light excitations were performed by using the second harmonic (532 nm) of a Nd:YAG laser (Spectra-Physics, Quanta-Ray GCR-4, full width at half maximum Ϸ 5 ns).…”

Section: Methodsmentioning

confidence: 99%

Primary charge-recombination in an artificial photosynthetic reaction center

Kobori

Yamauchi

Akiyama

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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101

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Photoinduced primary charge-separation and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-separation processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-separated state. The electronic coupling is found to be smaller by Ϸ40% than that for the primary charge-separation. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by configuration interaction between 3 (b1u,b3g) and 3 (au, b3g) electronic states of the free-base porphyrin.electronic coupling ͉ long-range electron transfer ͉ molecular orbital symmetry T o construct nanoscale devices or molecular wires that efficiently convert photon energies to chemical potentials, extensive studies have produced photoinduced, long-distance charge separation (CS) states by using donor (D)-acceptor (A)-linked multiarray systems that mimic natural photosynthesis (1-11). In each intramolecular, sequential electron transfer (ET) step, an electron or a hole tunnels between D and A over substantial distances (Ͼ10 Å) as in the natural photosynthetic reaction centers (12). From Marcus theory (13)(14)(15)(16)(17)(18)(19)(20), the ET rate constant (k ET ) is described (in the weak coupling regime) aswhere -h is Planck's constant divided by 2 and V, the electronic coupling matrix element, represents the electronic tunneling interaction between D and A. FCWDS denotes Franck-Condon weighted density of states parameterized by the reorganization energy ( ) and the driving force (Ϫ⌬G) for the ET reaction. The FCWDS term is given by (4where k B is the Boltzmann constant and T is the temperature. Although for several long-range ET systems, V has been interpreted in terms of a superexchange coupling model (21) that bridges the redox sites (22-28), much remains to be resolved regarding the tunneling mechanism (29-34). In addition, characterizations of V will be useful for designing molecular electronic devices (35). Key to efficient photoinduced CS is prevention of the energywasting charge recombination (CR) characterized in part by V. Although V and have been investigated for various ET steps by experimentally measuring k ET s in systems such as D 2 -D 1 -A triad (7, 9, 36) and protein complexes (24,25,34), primary CR,-A has escaped elucidation, especially in the efficient stepwise CS systems because the CR kinetics is hidden by subsequent CS processes,In the photosynthetic reaction center protein of Rhodobacter sphaeroides, which contains as cofactors a special pair (P), two bacteriopheopheophytins (H A and H B ), and two quinones (Q A and Q B ), the CR kinetics of the primary CS state of P ؉• H A Ϫ• Q A cannot easily be observed, because H A Ϫ• 3 Q A forward ET (Ϸ200 ps) is much faster than the primary CR (10-20 ns, which has b...

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“…Another in-house program was used to simulate the spectra as sums of Gaussian curves. Chemically induced dynamic electron polarization (CIDEP) effects ,,− were not taken into account by this program.…”

Section: Methodsmentioning

confidence: 99%

Study of PhotoinducedN-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

et al. 2004

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The photoreduction of aromatic nitro compounds by alcohols is a well-known reaction; however, the first stages of its mechanism remain controversial. This study aims at characterizing the "primary" radicalar transients involved in this reaction by EPR spectroscopy. Laser flash photolysis (lambda = 266 nm) of nitrobenzene, 5-nitrouracil, p-nitroacetophenone, o-propylnitrobenzene, and 2-nitroresorcinol in ethylene glycol was followed by time-resolved EPR spectroscopy. In all reported TR-EPR spectra, except those obtained from the photolysis of 2-nitroresorcinol, the key intermediate N-hydroxy-arylnitroxide radicals (ArNO*OH, 1-4) could be identified unambiguously. In 2-nitroresorcinol, the radical anion (ArNO*O(-), 5) and a sigma iminoxy radical (6) were observed, and a third radical (7) remains unidentified. These observations indicate that two radicalar mechanisms (by H* transfer and by electron transfer) are competing in the photoreduction mechanism. The attribution of the EPR spectra was helped by DFT calculations of the hyperfine coupling constants (hcc's).

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Computation of Time-Resolved EPR Spectra of Systems Exhibiting Electron Spin Polarization Complicated by Magnetization Transfer

Cited by 10 publications

References 26 publications

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

Primary charge-recombination in an artificial photosynthetic reaction center

Study of PhotoinducedN-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

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